Abstract: A recently discovered structure, dubbed as PC~217.96+32.3, is currently the
highest redshift Coma cluster analog at $z=3.78$, observed when the universe was
only 1.6 billion years old. A sensitive narrow- band imaging survey was
conducted over a 1.2deg$times.6deg ($approx 150times75$ comoving Mpc) region
enabling a complete mapping of the large-scale structure surrounding this
`protocluster' traced by Ly$alpha$ emitters (LAEs). Based on the existing
imaging and spectroscopic data, we have firmly established that: (1) the LAE
overdensity is one of the largest known, with an estimated combined mass of
$>10^M_odot$, which will evolve into a Coma-like cluster in the
present-day universe; (2) the LAE selection is very robust; and (3) the
kinematics of the known members suggests that the protocluster may be in the
process of coalescing. The distribution of LBG candidates over the full field
is remarkably similar to that of LAEs, suggesting that the two populations trace
the same large-scale structure. Intriguingly, near the core of the
protocluster, the LBG distribution is more extended than that traced by the LAEs
by $approx-4~Mpc (physical) further west. Based on this fact, combined with
the existing spectroscopic measurements, we speculate that the protocluster is
even larger than that traced by LAEs. The lack of LAEs there may be explained
if i) the structure extends at an angle from our viewpoint such that the western
end falls out of our LAE selection function; or ii) there exists an age gradient
or complex spatially varying formation history within the structure. The main
limitation has been to robustly measure the level and extent of the LBG
overdensity, which requires a higher source density to reduce shot noise and
cosmic variance. We request Mayall Mosaic 3 time to improve the sensitivity of
our existing optical dataset. The proposed data will play a central role in
quantifying the spatial extent of this extremely rare structure, which will
provide a remarkable crucible for the study of cluster formation.